Shortening device for relativistic electron pulses downstream of an accelerator



J. PINEL. 3,379,984 SHORTENING DEVICE FOR RELATIVISTIC ELECTRON April23, 1968 PULSES DOWNSTREAM OF AN ACCELERATOR 5 Sheets-Sheet 1 FiledMarch 10, 1965 INVENTOR:

J.P/NEL Qmmm 19 ATTORNEY April 23, 1968 J. PINEL 3,379,984

SHORTENING DEVICE FOR RELATIVISTIC ELECTRON PULSES DOWNSTREAM OF ANACCELERATOR 5 Sheets-Sheet 2 Filed March 10, 1965 INVENTOR Z J. P/IVELBY m &

,4. ATTORNEY April 23, 1968 J. PINEL 3,379,984

SHORTENING DEVICE FOR RELATIVISTIC ELECTRON PULSES DOWNSTREAM OF ANACCELERATOR 3 Sheets-Sheet 5 Filed March 10, 1965 INVENTOR ATTORNEYUnited States Patent 3,379,984 SHORTENHNG DEVICE FUR RELATIVESTTCELECTRGN PULSES DQWNSTREAM 9F AN ACCELERATOR Jacques Pine], Paris,France, assignor to CSF-(Iornpagnie Generals de Telegraphic Sans Fil,Paris, France Filed Mar. 10, 1965, Ser. No. 438,656 Claims priority,application France, Mar. 13, 1964, 967,266 16 Claims. (Cl. 328-230)ABSTRACT OF THE DISCLQSURE An electron accelerator installationcomprising an accelerator for accelerating electrons to relativisticvelocities and for delivering pulses containing a spatial sequence ofbunched electrons having successively decreasing energies, and a magnetpositioned on the path of said electrons downstream of the output of theaccelerator for shortening the duration of each of said pulses. Theaccelerated electrons propagate in the gap between the two pole piecesof the magnet. The induction in the gap is so chosen that afterdescribing their orbit the bunches reach simultaneously the same pointin space and are extracted from the gap at said common point.

The present invention relates to a device for shortening the pulses ofaccelerated particles.

The device of the present invention is intended to be used at the outputof a particle accelerator, more particularly of an accelerator ofelectrons carried to relativistic velocities, i.e., to velocities nearthe velocity of light, and wherein the accelerated electrons aredelivered in the form of pulses. The aim of the device in accordancewith the present invention is to shorten the pulses so as to enableproper handling in the course of measurements or experiments in thefield of nuclear physics.

It is known that owing to the inertia of circuits and sources ofparticles, it is hardly feasible to operate an accelerator in pulsedcondition with pulses shorter than several nanoseconds (for instance, 3nanoseconds or 3-10 seconds). During each of these pulses, a burst ofelectrons is delivered from the output of the accelerator which has avelocity substantially equal to the velocity of light, this burstforming a chaplet of washers separated by a distance equal to thewave-length of the high frequency wave which was used for operating theaccelerator, each of the washers containing substantially mono-energeticelectrons. However, the electrons in each washer have respectivelydifferent energies, the first washer having the highest energy and thefollowing washers having decreasing energies, according to asubstantially linear law as a function of their position within thechaplet. This decrease of energy is due to the fact that the wave whichhas accelerated the first washer has yielded to said washer a fractionof its energy, thereby keeping for accelerating the second washer onlyrelatively lower energy, etc. The length of the chaplet of washers mightnot be shorter than L=0.9 m. if the pulse duration could not be madeshorter than 3 nanoseconds, since, with the velocity of light 0:3 l0m./sec.,

Now, the phenomena to be measured or utilized in nuclear physics areassociated with durations which are of the same order of severalnanoseconds. It follows therefrom that the results of measurements arealtered when the pulses have a length of the same order, similar to theeffect of the internal resistance of a voltmeter or a-mrneter which maycause the measurements of voltage or current to be altered. Therefore,there is a need to shorten the pulses so that their length or durationcould be neglected when compared with the measuring time. The deviceaccording to the invention provides a solution of this problem.

According to the invention, there is provided at the output of a pulsedaccelerator a device called regrouper, which includes substantially amagnet having a gap through which is directed the chaplet of washersformed by the accelerated particles. The washers follow within the gap,about the lines of force of the magnet, circular orbits the radius ofwhich varies with the energy of each washer considered. The induction inthe gap is chosen so that different particles come or arrive, at thesame instant, substantially at a common point of their orbits; the inputchaplet of washers is then re-grouped at the output so as to form ashort bunch having a length appreciably smaller than the length of theinput chaplet of washers.

According to another feature of the invention, the magnet is sopositioned with respect to the input chaplet of washers that the axis ofthe chaplet is slightly inclined with respect to the plane perpendicularto the lines of force in the gap, the particles following thereby withinthe gap helicoidal paths having these circular orbits as projections ofthe paths on this perpendicular plane, with a view to enable theparticles to leave the re-groupcr without interfering with the particlesentering the same.

According to a further feature of the invention, the faces of the polepieces of the magnet are shaped to form a spiral or corkscrew staircasewinding about an axis passing through a peripheral point of the piece.

According to a further development of the invention, the lateral facesof the stairs of the staircase (the counterstairs) are formed bycylindrical surfaces orthogonal to the family of circles representingthe projections of the paths of the particles.

Accordingly, it is an object of the present invention to provide adevice which enables shortening of pulses derived from an accelerator toavoid the danger that the results of measurements or experiments ofnuclear physics are altered or rendered false owing to excessive pulselengths.

Another object of the present invention resides in a provision of amagnet having a novel shape and located between the output of a particleaccelerator and further utilization devices for the particles, therebyacting on the duration of the pulses of the particles in the sensefavorable to physical measurements or experiments.

A further object of the present invention resides in the provision of aninstallation of an electron accelerator, delivering pulses atrelativistic velocity, having a duration more appropriate to physicalmeasurements or experiments.

These and other objects, features and advantages of the presentinvention will become more obvious from the following description, whentaken in connection with the accompanying drawings which shows, forpurposes of illustration only, a manner of embodying the principle inaccordance with the present invention, and wherein:

FIGURE 1 is a basic schematic representation of an electron acceleratorinstallation in accordance with the present invention,

FTGURE 2 is a perspective view of the paths followed by the particles inthe re-grouper according to the present invention,

FIGURE 3 is a cross-sectional view of a bunch of particles leaving there-grouper, taken along line HIIII of FIGURE 1,

FIGURE 4 is a perspective exploded view of an example of a constructiveembodiment of the re-grouper in accordance with the present invention,

FIGURE 5 is a perspective assembled view of the same re-grouper,

FIGURE 6 is an end face view of one of the pole pieces of the re-grouperof FIGURE 4,

FIGURE '7 is a schematic diagram showing a further development of theaccelerator installation in accordance with the present invention, and

FIGURE 8 is a schematic diagram showing a still further development ofthe accelerator installation of the present invention.

FIGURE 1 shows an extremely schematic representation or" the device inaccordance with the invention, exclusively for the purpose of betterunderstanding of its operation.

The block 1 represents a conventional accelerator of electrons which arecarried to a relativistic velocity, this accelerator being in pulsedoperation. The electrons coming from the output of the acceleratorduring a pulse are grouped or re-assemhled within a voiurne of space 2,and form therein a chaplet of washers extending over a total length L,equal to the product of velocity near th velocity of light :3 10iii/sec. by the duration of the pulse. Assuming, for instance, that thewashers are circular, the envelope of the Chaplet would be cylindricalif the paths of electrons were parallel, but in fact there is always aslight divergence of these paths, and therefore the volume 2 is atruncated cone with small opening. This chaplet of length L moves withinthe space at the output velocity of the electrons, the diameter of thewashers increasing slightly and pro ressively during this movement dueto the aforementioned divergence so that the chaplet is containedbetween boundary paths 3 and 4. The energy of the subsequent washersdecreases between boundary levels E and E in proportion to the positionof each washers under consideration within the chaplet,

According to the invention, the chaplet of washers distributed in themanner specified above, enters into the gap of a magnet having polepieces shown in FIGURE 1 in projection in dash lines 5, and havingmagnetic lines of force perpendicular to the plane of the drawing. Theinduction in the gap is denoted by B (FIGURE 4). Within this gap, theelectrons begin to follow in the plane of the drawing circular pathshaving radii proportional to the momentum of each washer underconsideration, and hence proportional to the energies of the subsequentwashers, since their velocity is substantially the same, due to therelativistic acceleration of the electrons. Thus, the washer E follows acircle C an intermediate washer E follows a circle C and the last washerE follows a circle C FIGURE 1 shows a position wherein the washer E justarrived at the beginning of its circular path, while the washer IE atthe same instant has already progressed by a distance L along thecircumference of the circle (3;. 0

Intermediate washers occupy positions nearer E along their respectivecircles. The radii of the outermost and innermost circles C and C aredenoted respectively by R and r.

The result aimed at by the invention will be obtained if the washer Ehaving described the circle C returns to the same point as E at the sameinstant when the washer E having described the circle C returns to itsinitial position indicated in FIGURE 1. All intermediate washers wouldthen return substantially to the same point substantially at the sameinstant, due to proportionality between the energies and the positionsof the subsequent washers in the chaplet.

This condition corresponds to the numerical expression:

21rR27rl=L (1) As, on the other hand:

E l r E,, (2)

We have two equations from which R and r are calculated as follows:

Induction B required for a circle of radius R be described by electronshaving a given energy E is, accord ing to a known formula:

E B r wherein B is in Tesla (a Tesla being a unit of the internationalMKS system equal to 10,000 gauss), if E, is eX- pressed in mev. and R inmeters.

Thus, using one of the Expressions 3 or 4, it is found in my case:

21r(E E (B in Tesla, when E; and E in rnev. and L in meters).

Since all washers arrive simultaneously at the output point, it is seenthat if one causes the electrons to leave the gap at this very instant,all electrons remain grouped or re-assembled in a short bunch 6 (FIGURES1 and 3), having a length L substantially smaller than L, andcorresponding to the depth of a washer, which represents a fraction ofthe wavelength of the wave which was used for acceleration in theaccelerator 1. It may 'be admitted in practice that this reduced lengthwill correspond to a duration of the order of a fraction of onenanosecond, if the length L corresponded to 3 nanoseconds. This shortbunch 6 represents thus a shortening of the initial pulse and istherefore more readily utilizable for physical handling.

However, for purposes of a practical embodiment of the principleschematically shown in FIGURE 1, the device should be so constructedthat the output point of the short bunch 6 is prevented from coincidingin space with the input point of the chaplet of washers 2. Thiscoincidence could be troubling due to interference of input electronswith output electrons. The output point must be for this reason shiftedwith respect to the input point, in the direction parallel to lines offorce B. This shift can be attained by slightly tilting the magnet withrespect to the direction of the input chaplet of washers so that thedirection may have a slight slope to the plane perpendicular to thelines of force B. There are then no more circles described by theelectrons but turns of helices having circles C etc., as projectionsonto the plane normal to the lines of force B. FIGURE 2 shows aperspective View of a turn of helix 7 described by electrons havingenergy E and also of a turn of helix 8 described by electrons havingenergy E It is assumed here that both turns originate from a commonpoint A, and that the tangent to each of the turns 7 and 8, passingthrough the common point A and located in a tangent plane parallel tolines of force B, is common to both turns, i.e., has the same slopeangle with respect to the plane normal to these lines of force B. It isseen that the output point 8; or S is shifted with respect to the commoninput point in the direction of the lines of force B, and moreover thisshift is different for electrons having respective extremity energies Eand E while output points of electrons having intermediate energies areof course distributed between S and S It results therefrom that thecross section of the short bunch 6, taken perpendicular to the plane ofFIGURE 1 and parallel to the width d, has an elongated shape in thedirection of height h, as shown in FIGURE 3. In this cross section theelectrons are grouped following their energies progressively decreasingfrom the bottom upwards in FIG- URE 3, as symbolized by the crosshatching which is less and less dense in the upward direction.

FIGURES 4 and 5 show in perspective, respectively, an exploded and anassembled view of an example of structural embodiment of theabove-explained principles. In FIGURE 4, reference numerals 5 and 5' areused to designate two pole pieces corresponding to the projection 5 inFIGURE 1. The magnet is energized by coils 9 and 10, and the magneticcircuit is completed by a yoke 11.

In the gap between pole pieces 5 and 5' is located a guide 12 having aninput 13 intended to be connected to the evacuated enclosure of anaccelerator corresponding to the block 1 of FIGURE 1, and having anoutput 14 intended to be connected to the enclosure (not shown) of autilization device of the derived short bunch. This guide 12 is made ofa non-magnetic metal and has a helicoidal shape with variable crosssection, properly sized for passing all paths comprised between theextreme helices 7 and 8 of FIGURE 2. The radii R and r of these helicescan be calculated from Equations 3 and 4; for instance, if the length Lof the pulse 2 in FIGURE 1 is 0.9 meter, and if the energies of thefirst and the last With the same data, the induction B in the gap iscalculated from Equation 6:

=O.7 Tesla The input branch of the guide 12 is passed through a magneticchannel provided according to the invention in the body of the polepiece 5; the input of this channel is seen at 15 and the output at 16.In the same manner, the output branch of the guide 12 is passed througha magnetic channel provided in the body of the pole piece 5', with theinput at 17 and the output at 18. The fringing areas of the magnet arethereby traversed without disturbing the electron paths inside of theguide 12 before their arrival at the starting point of the helicoidalpath, or along their trajectory followed after having described thehelix. If to the contrary, these channels were manufactured separatelyfrom the pole pieces and thereafter introduced in the gap, theirpresence would disturb the distribution of induction in the gap.

The size of the guide 12 parallel to lines of force B should be, ofcourse, slightly larger than the height h in FIGURE 3. This height itcorresponds to the shift between points S and S in FIGURE 2, added tothe diameter of the washers in the chaplet 2, with consideration of thedivergence of the chaplet. The distance between points S and S issubstantially equal to:

where c: is the slope of the helix described by the guide 12, expressedin radians, and taken with respect to the plane normal to lines of forceB. On the other hand, this slope should be such that the output branchof the guide passes below the input branch, the thickness of the Wallsof the guide 12 being duly considered. For instance, with oc=0.04radians, the distance between the output points of electrons is21r(O.29-0.145) -0.04=0.036 rn.=3.6 crn.

Assuming the diameter of the washers in the chaplet 2 at the input intothe magnet to be equal to 0.4 cm. the height of the cross section of theguide 12 would be taken slightly greater than 4 cm. It is readilychecked that with a slope of 0.04 and a height of the order of 4 cm.,the output branch of the guide passes below the input branch thereof.

The size of the gap of the magnet should then be chosen slightly greaterthan this height with the addition of safety distances on both sides ofthe guide with respect to the metallic faces of both pole pieces. Forinstance, with a guide height of the order of 4 cm., the gap width couldbe chosen of the order of 6.5 cm.

With the output face 16 of the input channel located in the same planeas the input face 17 of the output channel, and with this plane disposedparallel to lines of force B and containing the normal line common tosaid lines of force and to the movement direction of electrons, themagnetic field distribution along the helicoidal paths followed by theelectrons between output face 16 and input face 17 must be progressivelyvaried or matched in order to avoid defocusing effects. This matchingcan be attained if the pole faces are shaped in the form of a staircasehaving stairs 19, 19", 19", etc., on the pole 5 and 26', 20", 20", etc.,on the pole 5'. These stairs each turn in the fashion of a corkscrewstaircase, having for its axis a vertical line passing through the planeof opening 16 or 17, respectively. The faces of the stairs aresubstantially perpendicular to the desired direction of the lines offorce B.

Each stair, when considered together with an opposite facing stair onthe opposed pole piece, forms a pair of pole pieces capable of beingassimilated to a partial magnet so that the electrons within the gapsuccessively traverse a series of such partial magnets joined by theirlateral faces.

These partial magnets, having gaps traversed successively by electrons,define magnetic fields which are uniformly distributed in each partialmagnet, and moreover equal in all magnets if the width of their gaps isthe same. Thus, the gap chosen, for example equal to 6.5 cm. asindicated above, will be maintained between each pair of facing stairs.

On the other hand, in order to avoid as much as possible a defectivefocusing, it is desirable that the lateral faces joining the partialmagnets be normal to the trajectories of electrons. This is why,according to a further development of the invention, the lateral facesof the stairs (counter-stairs), such as 21 or 22, are cylindricalsurfaces orthogonal to the family of circles representing projections ofhelices described by electrons. In FIG- URE 6, showing a face View ofthe pole piece 5, and indicating in dash line the circle C of FIGURE 1,it is seen that the lines or circles 22 representing the projection ofcounter-stairs of the staircase composed of stairs 20, 20", 20, etc.,are orthogonal to the circle C and have a common point A so that circles22 are orthogonal with respect to the family of circles mutually tangentat the point A.

If it is desired that the short bunch 6 shown in FIG- URES 1 and 3 beenergy-focused, i.e., that its height h be reduced to practicallypin-point size, it is possible to apply for this purpose a focusingdevice, known per se, for instance a system of two magnets 23, 24 withparallel faces (FIGURE 7), properly dimensioned and mutually spacedaccording to Well known teachings of prior art. The lines of force ofthese magnets must then be directed along the width [1, i.e.,perpendicular to lines of force B of the re-grouping magnet 5. Afterpassing through the magnets 23 and 24, the height h of the short bunch 6is reduced to practically pin-point size, and the cross section of thebunch becomes such as shown in 25.

Finally, the shift between points S and S could be reduced if betweenthe accelerator 1 and the re-grouper 9, 1G, 11 (FIGURE 8) there isinserted a deflecting magnet 26 having lines of force perpendicular onthe one hand to the initial direction of movement of the electrons, andon the other, to the induction in the re-grouper. By means of thismagnet 26, the paths of electrons with different energies are spread inthe plane perpendicular to its lines of force, so that the slope ofthese paths varies inversely with energy. It results therefrom thatwhile in FIGURE 2 the helices 7 and 8 had the same slope but differentpitches, the interposition of the magnet 26 modifies the helical pathsso that their slopes becomes different, the helix 8 having a smallerslope. The difference between their pitches would then be reduced andcould even decrease to zero, both helices '7 and 8 having then the samepitch, and both points S and S coming into coincidence. The crosssection of the short bunch 6 in FIGURE 3 would then become circular asthe cross section of the truncated cone 2 in FIGURE 1. The gap allyparallel, by means of a second deflecting magnet 27,

acting oppositely to the magnet 26.

While I have shown and described one embodiment in accordance with thepresent invention, it is understood that the same is not limited theretobut is susceptible of numerous changes and modifications as known to aperson skilled in the art. Especially, the pole pieces of the regroupingmagnet could be incorporated in the evacuated enclosure. The system offocusing magnets shown in FIGURE 7 could be replaced by a system ofquadru-polar lenses or similar focusing means. The induction of there-grouper could be made adjustable for adapting the device to operatewith different maximum and minimum energy levels. The re-grouper couldbe made orientable in space, to vary the impact slope of the electrons.The described device is of course applicable to particles other thanelectrons, if the accelerator of said particles is powerful enough forcarrying them to relativistic velocities. Therefore, I do not wish to belimited to the details shown and described herein but intend to coverall such changes and modifications as are encompassed by the scope ofthe appended claims.

I claim:

1. In a particle accelerator installation, an accelerator operative toaccelerate electrons to relativistic velocities and to deliver theelectrons in the output thereof in the form of pulses containing aspatial sequence of electrons having successively decreasing energies,and means positioned along the path of said electrons downstream of theoutput of said accelerator for shortening the duration of each of saidpulses by grouping electrons contained in a respective pulse intorelatively reduced spatial length.

2. In a particle accelerator installation, an accelerator operative toaccelerate electrons to relativistic velocities and to deliver theelectrons in the output thereof in the form of pulses containing aspatial sequence of electrons having successively decreasing energies,and means posi tioned along the path of said electrons downstream of theoutput of said accelerator for shortening the duration of each of saidpulses by grouping electrons contained in a respective pulse into arelatively reduced spatial length, said means including re-groupingmagnet means producing lines of force and having a pair of pole pieceswith a gap therebetween traversed by said accelerated pulses, with saidelectrons describing orbits about the magnetic lines of force in saidgap, the induction in said gap being so chosen that after describing oneorbit substantially all electrons are re-grouped at a given instant in asubstantially common point of space, and means for extracting saidre-grouped electrons from said gap at said common point.

3. The combination according to claim 2, wherein said orbits aresubstantially closed.

4. The combination as claimed in claim 2, wherein the induction in saidgap is chosen substantially according to the expression:

wherein:

B is the induction, in Tesla, E the energy of electrons having thehighest energy,

in mev., E,,, the energy of electrons having the lowest energy,

in mev., L, the length of an accelerated pulse, in meters. 5. Thecombination as claimed in claim 2, wherein said re-grouping magnet meansis positioned in space so that the impact direction of an acceleratedpulse is lit slightly inclined with respect to the plane perpendicularto the lines of force in said gap.

6. The combination as claimed in claim 5, further comprising helicoidalguide means for said electrons within said gap.

7. The combination as claimed in claim 6, wherein said guide means has avariable width along the path of said electrons, said width firstprogressively increasing from the input of said electrons and thenprogressively decreasing toward the output thereof.

8. The combination as claimed in claim 5, wherein the faces of said polepieces are shaped to form a corkscrew staircase turning about an axispassing through the periphery of both pieces.

9. The combination as claimed in claim 8, wherein the lateral faces ofthe stairs of said staircase are substantially cylindrically shaped,said cylindrical surfaces being orthogonal to the family of circlesrepresenting projections on said pole pieces of the paths of saidelectrons in said gap.

19. The combination as claimed in claim 5, further comprising magneticchannel means in the body of each of said pole pieces, respectively, forthe input and for the output of said electrons in said gap.

'11. In a particle accelerator installation, an accelerator operative toaccelerate particles to relativistic velocities and to deliver theparticles in the output thereof in the form of pulses containing aspatial sequence of particles having successively decreasing energies,and means positioned along the path of said particles downstream of theoutput of said accelerator for shortening the duration of each of saidpulses by grouping particles contained in a respective pulse into arelatively reduced spatial length, and focusing means positioned alongthe path of said shortened pulses.

12. In a particle accelerator installation, an accelerator operativetoaccelerate particles to relativistic velocities and to deliver theparticles in the output thereof in the form of pulses containing aspatial sequence of particles having successively decreasing energies,and means positioned along the path of said particles downstream of theoutput of said accelerator for shortening the duration of each of saidpulses by grouping particles contained in a respective pulse into arelatively reduced spatial length, and deflecting means for theparticles between said accelerator and said shortening means.

13. In a particle accelerator installation, an accelerator operative toaccelerate particles to relativistic Velocities and to deliver theparticles in the output thereof in the form of pulses containing aspatial sequence of particles having successively decreasing energies,and means positioned along the path of said particles downstream of theoutput of said accelerator for shortening the duration of each of saidpulses by grouping particles contained in a respective pulse into arelatively reduced spatial length, and deflecting means for theparticles between said accelerator and said shortening means, and seconddeflecting means for correcting the effects of said first-nameddeflecting means, said second deflecting means being positioned alongthe path of said shortened pulses.

14. In a particle accelerator system having an accelerator producing inthe output thereof pulses of accelerated electrons moving atrelativistic velocities, each pulse consisting of a chaplet of washersseparated by predetermined distances, the improvement essentiallyconsisting of a corrective system disposed along the path of theaccelerated electrons downstream of the accelerator output includingre-grouping means having input means and output means for re-groupingthe washers in each chaplet so that the length of a respective chapletin said output means is considerably shorter than the length thereofcorresponding to the pulse length in said input means.

15. In a particle accelerator system having an accelerator producing inthe output thereof pulses of acelerated particles moving at relativisticvelocities, each pulse consisting of a chaplet of Washers separated bypredetermined distances, the improvement essentially consisting of acorrective system disposed along the path of the accelerated particlesdownstream of the accelerator output including i e-grouping means havingmagnet means, input means and output means for re-grouping the washersin each chaplet so that the length of a respective chaplet in saidoutput means is considerably shorter than the length thereofcorresponding to the pulse length in said input means, said magnet meanseffectively producing paths of different lengths followed by each washerproportional to the average energy level thereof.

16. In a particle accelerator system having an accelerator producing inthe output thereof pulses of accelerated particles moving atrelativistic velocities, each pulse consisting of a chaplet of washersseparated by predetermined distances, the improvement essentiallyconsisting of a corrective system disposed along the path of theaccelerated particles downstream of the accelerator output includingre-grouping means having magnet means, input means and output means forre-grouping the washers in each chaplet so that the length of arespective chaplet in said output means is considerably shorter than thelength thereof corresponding to the pulse length in said input means,said magnet means effectively producing paths of different lengthsfollowed by each washer proportional to the average energy level thereofin such a manner as to produce coincidence of all washers in arespective chaplet in said output means.

References Cited UNITED STATES PATENTS 2,395,647 2/1946 Strobel 331802,872,574 2/1959 McMillan et al 328-234 3,031,596 4/1962 Leboutet et al3l3 84 3,089,092 5/1963 Plotkin et al 328155 3,268,729 8/1966 Adachi eta1. 250-845 IAMESXV. LAWRENCE, Primary Examiner.

V. LAFRANCHI, Assistant Examiner.

